The general procedure for drilling an oil or gas well includes setting a relatively short string of casing, normally referred to as a "conductor pipe," in a borehole soon after the start of an operation. The annulus between the outside of the conductor pipe and the inside of the borehole is typically filled with a mixture of portland cement and water. A slightly smaller hole is then drilled below the conductor pipe and another string of casing, typically referred to as the "surface casing," is run and set. Surface casing may be set as shallow as 500 feet or as deep as 6,000 feet.
The next string of casing set in the borehole is usually referred to as an "intermediate string" or a "protection string" if it is a deep hole. The next string might be used to case a hole to its total depth, in which case the string would be referred to as the "production string." This disclosure relates primarily to setting the surface casing.
The principle objective in cementing surface casing is to secure the casing in the borehole so that the vibration and hammering of the whipping drill pipe will not damage the casing. If surface casing is not properly set, the bottom joints of the casing may break free and drop, obstructing further drilling. Another reason for properly setting the surface casing is to seal off communication of the borehole with upper water sands, preventing the loss of heavier drilling fluids that might be required for drilling the deeper portion of a hole. Basically, the purpose for properly setting the surface casing is to secure the casing and to prevent communication between water, oil and gas producing zones and other porous zones.
An objective of this disclosure is to provide an improved process for grouting surface casing in an oil or gas well borehole.